Dual Trim Switches?

On Sep 28, 9:15 am, Christopher Brian Colohan
wrote:
After flying Tomahawks and Citabrias, I took my first lesson in a
Cessna 172 last night. I asked many questions, but I had one question
which nobody in my flying club (including the chief pilot) knew the
answer to when I was the Why does the electric elevator trim have
two switches (both of which must be depressed) on the yoke instead of
one?

I assume this is a recent model 172. To avoid a stuck switch (or
shorted switch) from running the trim. In the preflight checklist that
Cessna provides one of the tests under the "trim test" section is to
ensure that the trim does not move with either of the switches is
moved by itself. Additionally you should ensure that the electric trim
does not work at all when the auto-pilot cut-off is held down.

The best theory I could come up with was "to avoid runaway trim if one
switch stuck", but that is not very satisfying...

Anyone know why?

Chris

(Another question: 13 fuel drains? What were they thinking??? But I
am pretty sure the answer is "the lawyers designed that part, not the
engineers.")

No one really knows. Some believe that there is one for every lawsuit
someone brought against Cessna because they forgot to drain the tanks.
Few actually believe that Cessna designed the tanks to collect water
at that many points. In anycase, we just drain them all.

-Robert, CFII

"Robert M. Gary" wrote

No one really knows. Some believe that there is one for every lawsuit
someone brought against Cessna because they forgot to drain the tanks.
Few actually believe that Cessna designed the tanks to collect water
at that many points. In anycase, we just drain them all.

From the pictures provided (by a link) in an earlier post, it is obvious
(with the help of some added color agent) that there are, or could be, some
very big problems if the tank is not drained at all of the provided drains.

Everyone knows that the airplane is a group of compromises flying in close
formation. It seems as though the tank design is a _big_ compromise, for
some reason.

It is hard for me to understand how Cessna could come up with such a poor
design. I do realize that a wet tank has to have internal structures in
place, with weight and strength being two of the most important design
criteria, but it does seem like it should be possible to leave some pass
through areas to let gas and water flow freely from bay to bay.

It might have cost a little more, but when you are spending, what, over 200
AMU's to buy an airplane, should it not be possible to include some design
improvements that costs a little more, than take the easy way out with
nearly a dozen added (should be) unnecessary drains?

If a tank design like that is Cessna's normal way of dong things, I would
think Columbia's future owners have some genuine concerns.
--
Jim in NC

It might have cost a little more, but when you are spending, what, over 200
AMU's to buy an airplane, should it not be possible to include some design
improvements that costs a little more, than take the easy way out with
nearly a dozen added (should be) unnecessary drains?

They're not just unnecessary -- they're dangerous. That is 13
separate points of failure that should not be in that wing.

I've had several quick drains leak over the years -- one quite badly
-- now imagine 13 of them! All plumbing fittings, by their very
nature, will eventually leak. These will, too.

There's really no other way to put it: Cezzzna really screwed the
pooch with their wing design.
--
Jay Honeck
Iowa City, IA
Pathfinder N56993
www.AlexisParkInn.com
"Your Aviation Destination"

Recently, Jay Honeck posted:

It might have cost a little more, but when you are spending, what,
over 200 AMU's to buy an airplane, should it not be possible to
include some design improvements that costs a little more, than take
the easy way out with nearly a dozen added (should be) unnecessary
drains?

They're not just unnecessary -- they're dangerous. That is 13
separate points of failure that should not be in that wing.

I've had several quick drains leak over the years -- one quite badly
-- now imagine 13 of them! All plumbing fittings, by their very
nature, will eventually leak. These will, too.

There's really no other way to put it: Cezzzna really screwed the
pooch with their wing design.

Our club's 172 SP had those drain points. As I recall, only 10 of them
were in the wing, with the other 3 on the cowling. I can only imagine the
pretzel fuel flow that made that many drains necessary. I agree with the
OP that suggested that Cessna's lawyers designed that aspect of their
planes.

Neil

On Tue, 02 Oct 2007 09:56:00 GMT, "Neil Gould"
wrote:

Recently, Jay Honeck posted:

It might have cost a little more, but when you are spending, what,
over 200 AMU's to buy an airplane, should it not be possible to
include some design improvements that costs a little more, than take
the easy way out with nearly a dozen added (should be) unnecessary
drains?

They're not just unnecessary -- they're dangerous. That is 13
separate points of failure that should not be in that wing.

I've had several quick drains leak over the years -- one quite badly
-- now imagine 13 of them! All plumbing fittings, by their very
nature, will eventually leak. These will, too.

There's really no other way to put it: Cezzzna really screwed the
pooch with their wing design.

Our club's 172 SP had those drain points. As I recall, only 10 of them
were in the wing, with the other 3 on the cowling. I can only imagine the
pretzel fuel flow that made that many drains necessary. I agree with the
OP that suggested that Cessna's lawyers designed that aspect of their
planes.

AIUI, it's because they're bladders and folds on the bottom could
gather water. If it was a wet wing they wouldn't need as many, like
the older models.

For what it's worth.

When I first checked out in an SP, the CFI told me Cessna had been
sucessfully sued when a pilot argued the tanks could not be sumped correctly
unless the aircraft was parked perfectly level.

Now you can drain the lowest point in the tank, no mater how or where you
park your SP.

Maxwell wrote:
For what it's worth.

When I first checked out in an SP, the CFI told me Cessna had been
sucessfully sued when a pilot argued the tanks could not be sumped correctly
unless the aircraft was parked perfectly level.

Now you can drain the lowest point in the tank, no mater how or where you
park your SP.

I would tend to doubt that version of events. The 13 drain points were
introduced when Cessna restarted production on the redesigned 172R back in
'96. The drains were in the new version from the start, so I doubt Cessna
could have been sued. Also, we probably would have heard about it here.

Previous versions of the 172 ('84/'85) had wing tanks with no obstructions
in the bottoms. The new version had wet wings, which is what prompted the
installation of drains outboard of each rib in the tank area.

John Galban=====N4BQ (PA28-180)

--
Message posted via AviationKB.com
http://www.aviationkb.com/Uwe/Forums...ation/200710/1

"Peter Clark" wrote

AIUI, it's because they're bladders and folds on the bottom could
gather water. If it was a wet wing they wouldn't need as many, like
the older models.

But they are wet wings, in question, AFAIK. Did you see the picture that
someone posted in a link? Each rib was forming a little damn that could
possibly trap a small amount of water, so a drain was installed to drain any
trapped water, in each bay.
--
Jim in NC

On Wed, 3 Oct 2007 17:50:35 -0400, "Morgans"
wrote:


"Peter Clark" wrote

AIUI, it's because they're bladders and folds on the bottom could
gather water. If it was a wet wing they wouldn't need as many, like
the older models.

But they are wet wings, in question, AFAIK. Did you see the picture that
someone posted in a link? Each rib was forming a little damn that could
possibly trap a small amount of water, so a drain was installed to drain any
trapped water, in each bay.

The airfraft in that link is a 1982 172P, not a new (post-restart)
172R/S, 182T, etc with bladders in the wings. It's been a while (and
the only older model 172 I've flown is an M model) but I thought the
pre-restart aircraft only had 2 sumps at the wing root and a stariner
drain pull-thingie in the oil door?

You have the same issue as that 172P with the Malibu. Since they're
wet wings, the only path water has to flow underneath the ribs is the
minute clearance between the ribs and the lower wing skin. At least
the Malibu has some pretty significant dihedral. But if the aircraft
hasn't been sitting for (I'm not going to try and test it) some large
number of hours, unless you're pumping in almost pure water I doubt a
significant sample of water would show up in the 1 sump at the wing
root from fueling during quick (1hr or less) refueling stops during
multi-leg flights.

Help

I flew a 'Top Hat' four way trim switch for thousands of hours and
never saw a second one. Anyone know where it was located?

Our emergency procedure for run away trim was to pull the circuit
breaker(s).

Big John

*************************************************


On Fri, 28 Sep 2007 09:43:28 -0700, "Robert M. Gary"
wrote:

On Sep 28, 9:15 am, Christopher Brian Colohan
wrote:
After flying Tomahawks and Citabrias, I took my first lesson in a
Cessna 172 last night. I asked many questions, but I had one question
which nobody in my flying club (including the chief pilot) knew the
answer to when I was the Why does the electric elevator trim have
two switches (both of which must be depressed) on the yoke instead of
one?

I assume this is a recent model 172. To avoid a stuck switch (or
shorted switch) from running the trim. In the preflight checklist that
Cessna provides one of the tests under the "trim test" section is to
ensure that the trim does not move with either of the switches is
moved by itself. Additionally you should ensure that the electric trim
does not work at all when the auto-pilot cut-off is held down.

The best theory I could come up with was "to avoid runaway trim if one
switch stuck", but that is not very satisfying...

Anyone know why?

Chris